Reprinted with permission from November 2014 QST ARRL, the national association for Amateur Radio® www.arrl.org

Eric P. Nichols, KL7AJIt’s easy to overlook the less glamorous electromechanical technology of motors and actuators in the ham shack. Motors can be mystifying, solenoids — annoying, and steppers can be staggering. We totally ignore them, until they fail to work in the proper manner. This article will take some mystery out of these devices, and describe some of the wonderful new developments in electromechanical gadgets that will infil-trate the ham shack of the future.

ElectromagnetsMore than any other device, the electro-magnet is responsible for moving things around in your ham shack, whether it’s the rotator that twirls the cubical quad atop your tower, or the diaphragm that wobbles air between your dynamic headset and your ears. We’d be hard pressed to get much hamming done without electromagnets. Despite developments in electrostatic, piezoelectric, and fluidic actuators, the electromagnet still reigns supreme among electromechanical devices.

Solenoids Most mechanically meaningful electro-magnets appear as some form of solenoid, which is a coil of copper wire wrapped around some form of core. The core is nearly always an iron-bearing (ferrous) material, which serves to increase the permeability of the solenoid. The core con-centrates the field, making a more powerful electromagnet for a given amount of occu-pied space. The iron core is typically made of laminated soft iron to minimize hyster-esis effects, which tend to make an elec-tromagnet a permanent magnet. Although permanent magnets have an important role in electromechanical devices, you don’t want unintentional permanent magnets.

A solenoid is often used in conjunction with an armature, a movable slug or lever

Motors and Mechanisms in the Ham Shack

Your shack contains many electromechanical devices that whir, rattle, clatter, and occasionally bang. Here’s a

brief guide to what they are and what they’re for.

An electric motor.

of soft iron in proximity to the solenoid. An energized coil attracts the armature. The relay, or electromechanical switch, is one of the simplest applications of the solenoid.

Relays range from nearly microscopic reed relays to monstrous contactors used in power substations. Solenoids can be extremely fast and powerful, but they are somewhat difficult to control in a linear fashion. Solenoids are gen-erally either fully on or fully off. For a given coil current, the magnetic pull against the armature changes radi-cally with distance. For this reason, straightforward solenoids are seldom used in robotic devices or with other electromechanical devices that need a smooth motion over a wide range. For ro-bots, we need a more sophisticated actuator configuration of the electromagnet.

Motors and RotorsThe next level of electromagnetic device complexity is the motor. There are count-less varieties and configurations of motors; they all have some common elements. Motors can be broadly categorized as dc or ac motors. Commonly, dc motors often use permanent magnets. Generally, ac mo-tors are synchronous, meaning their speed

of rotation depends on the ac line frequency. Some motors, such as induction motors, are “semi-synchro-nous” in that their speed depends on both the line frequency and line voltage, as well as on the load.

Some form of commutation is needed with dc motors and generators. A commutator is a device for periodically re-versing the polarity of the armature current. Commutators are generally troublesome

Before GPS, the 60 Hz power grid line frequency was by far

the most accurate frequency standard

available to the average ham.

QST® – Devoted entirely to Amateur Radio www.arrl.org Reprinted with permission from November 2014 QST

you turn the shaft of one selsyn, the other one turns by the exact same amount. These were once used as rotational transducers on countless post-war ham antenna rota-tors. Earlier they were used extensively on radar systems or anywhere else you needed remote indication of a rotational value.

Most selsyns are designed to use 440 Hz ac, so if you plan on using one you might

want to build a small 440 Hz inverter. Internally, the selsyn is a modified three-phase motor with a three-phase stator, and a two-phase ar-mature. Each of these legs is connected to the matching legs on the complementary

device. Power is applied in parallel only to the armatures.

Stepper MotorsThe stepper motor may be the greatest departure from conventional motor design, and was made possible by very powerful and relatively cheap rare earth magnets. With the proper drive electronics and pro-gramming, the stepper motor is capable of very rapidly and repeatedly returning to a precise position. They are the heart of most computer printers and many computerized machine tools. However, a stepper motor can lose count of steps occasionally, which means some sort of reset or reboot function is needed in the more critical applications.

is an ac semi-synchronous induction motor with two windings at 90-degree angles. Switching a proper capacitance into one of the windings creates a phase shift in the motor, causing the motor to run either clockwise or counterclockwise at a con-stant speed. You (or the controller) need some sort of feedback to tell you when the rotator is pointing where you want it. The simplest directional detector is a potentiometer attached to the rotator output. This is the in-dependent reference. Finally, you need a comparator, which is another potentiometer in the control box that compares the voltage from the reference potentiometer in the rotator as-sembly with the position of the “big knob.” When these two values are equal, the volt-age to the motor shuts off. The motor may reverse briefly or it overshoots.

Servo loops are also used in automatic antenna tuners, although the trend favors using stepper motors, which are easier to design than tricky feedback loops.

Selsyns and SynchrosThe synchro or selsyn (for self-synchroniz-ing motor), once very prevalent, is one of the coolest devices ever created. A selsyn set consists of two motor-like devices con-nected by five wires. Neither one of them rotates when you apply power. However, if

and require maintenance, which is why most cars converted from dc generators to alternators in the late 1950s. Recently, brushless dc motors (where the commu-tator has been replaced with a solid state switch) have been appearing in ham radio and computer fans. The brushless dc motor is not a synchronous motor.

The surprisingly accurate old-fashioned ac wall clock uses a synchronous ac motor. This is because the 60 Hz line frequency of the US power grid is atomic clock accurate when averaged over long periods. In fact, before global positioning satellites (GPS), the 60 Hz power grid line frequency was by far the most accurate frequency standard available to the average ham. So, don’t toss out your plug-in wall clock just yet.

Servo with a SmileThe servomotor or servomechanical device is a marvel of the last century. A servomo-tor has a feedback mechanism (loop) to reduce or compensate for mechanical er-rors somewhere in the system. The servo loop is analogous to the negative feedback amplifier. In fact, the math describing them is identical. Feedback theory became for-malized after World War II.

There are several types of motor feedback loops that control speed, position, phase, or a combination of them. The nearly universal phase-locked loop (PLL) used in modern transmitters and receivers is actually derived from motor speed servo control. Any type of servo loop requires an independent reference, a comparator, and a controller. A typical antenna rotator, which uses a positional servo loop, employs all three items.

The controller is the bi-directional motor, which is often a split-phase ac motor. That

A servo motor.

A solenoid. [Image courtesy of Direct Industry]

More than any other device, the electromagnet is responsible for

moving things in your ham shack.

Reprinted with permission from November 2014 QST ARRL, the national association for Amateur Radio® www.arrl.org

For updates to this article, see the QST Feedback page at

www.arrl.org/feedback.

ARRL member Eric Nichols, KL7AJ, has written numerous articles for many Amateur Radio and electronics experimenter publications over the past 30 years. He can be reached at PO Box 56235, North Pole, AK 99705 or kl7aj@arrl.net.

An electric motor armature.

A piezoelectric actuator. [Image courtesy of Direct Industry]

A stepper motor. [Image courtesy of Automation Technology]

A servo-motor, on the other hand, won’t lose its place.

The linear actuator is a useful variation of the stepper motor. It is basically a stepper motor driving a precision jackscrew. These are capable of tremendous amounts of pushing and pulling force and extremely high accuracy. Most micropositioners used in microsurgery and micro-manufacturing use some sort of stepper linear actua-tor. These require careful programming. Linear actuators are often used in robotic systems.

Small-scale ForcesThere are electromechanical methods that do not rely on magnetism to move things. Most hams are familiar with piezoelectric properties of quartz crystals. When you apply an electric field to a hunk of quartz, it warps slightly. Until recently, this amount of warping was considered too minuscule to do any real work. However, piezoelec-tric actuators are now used for performing surgery on the cellular level. Only slightly less refined than the piezoelectric actuator is the magnetostrictive actuator. Magneto-striction is the distortion of a metal by an electric field, as used in the famous Col-lins mechanical filters. Magnetostrictive actuators are used in micro-manufacturing as well.

Finally, we have electrostatic actuators. These have the least amount of force avail-

are not going to go away any time soon. There are certain things you just can’t do with silicon and software.

able, but are very cheap and are useful for many special applications. Electrostatic transducers have been used for a long time as components of loudspeakers and headphones.

Not all electromechanical devices are even solid. For example, spiders use hydraulic pressure to extend their limbs, as they lack extensor muscles. And in a similar fashion, tiny robots use tiny hydraulic systems. Special MHD (Magneto Hydro-Dynamic) fluids can be pumped directly with tiny electric currents instead of master cylin-ders, which can result in robots that are smaller — and creepier — than spiders. Now you know.

Back on EarthIt will probably be a while before armies of spider-droids are going to have much appli-cation in the typical ham shack. However, we need to realize that mechanical devices